The shade ring correction for measurements of diffuse irradiance under clear skies

Abstract

Observed and estimated values of the shade ring correction in the measurement of diffuse irradiance are compared for a sample of days which were cloud-free or almost so. Corrections are determined observationally from measurements of diffuse irradiance using both a shade ring and an occulting disc, which are recorded every minute. Estimated corrections are calculated (a) from numerical integration of analytic radiance distributions and (b) from empirical formulae. The former method gives results that are sensitive to the choice of radiance distribution. Method (b) does not account for the variation in corrections throughout the day, but is adequate for most periods. Errors due to a misalignment of the shade ring are examined, using calculations from analytic radiance distributions.